Officials on Tuesday morning had not determined what started the fire, which began after 7:30 p.m. Monday. Firefighters did not clear the scene until 4:30 a.m. Tuesday.

During the fire, police closed U.S. Route 1 from Maple Avenue to the Maine Medical Center Campus.

Thurlow said state fire marshals and local investigators would be on the scene all day Tuesday, conducting interviews with tenants of the 24,000-square-foot office complex, which is across the street from Lois’ Natural Marketplace at the intersection with Portland Farms Road.

Officials could not confirm whether any employees or customers were in the building at the time of the fire, and Thurlow said they hope to determine so during their investigation Tuesday.

The Fire Department estimated at least 17 or 18 businesses occupy the space, including local engineering firm Northeast Civil Solutions, two massage centers, two medical practices, two travel agencies, a hair salon, and a legal center.

All the businesses experienced significant interior damage, and most were destroyed and displaced.

“There was plenty of damage to go around,” Thurlow said.

More than half a dozen neighboring fire departments came to the aid of Scarborough crews over the course of the evening, he said.

The town assessor’s office said Scarborough Commons was built in 1984, and several additions to the building were made in succeeding years. Fire officials said it may have been those structural additions, and the building’s size, that made the fire difficult to control.

Thurlow said there were no injuries, although three firefighters were sent to the hospital as a precaution for heat exhaustion. All three were later released.